This page provides the necessary basics to understand the behavior of Crown-of-Thorns Starfish (COTS) and the population explosion phenomenon commonly known as COTS outbreak or COTS infestation that has devastating consequences for the coral reefs of the Indo-Pacific.
Among the broad range of large-scale disturbances that affect Indo-Pacific coral reefs, the coral-eating starfish Acanthaster planci (“crown-of-thorns”, COTS hereafter) is a major cause of coral reef destruction, whose impact is quantitatively comparable to cyclones. Monitoring and managing COTS outbreaks with the involvement of local villagers and low-cost natural methods can successfully protect key portions of reefs and help them face other threats.
Ecology of COTS
Crown-of-thorns, a coral predator
Acanthaster planci (“crown-of-thorns”, COTS) is a very large starfish species observed throughout the Indo-Pacific region on coral reefs. Specimens reach up to 70 cm in diameter and 3 kg in size; colour varies according to region. This species has a large number of arms (usually 16 to 18) and carries on its upper face long spines coated with highly toxic steroid venom that inflict especially painful stings. Though they appear to move slowly, they exhibit an active prey-searching roaming behavior (up to 20m/day). They can potentially cross from one reef to another by moving across deeper section of the seabed.
COTS have a strong capacity to recover from injuries, with the ability to grow back severed arms. If a COTS is cut in two pieces, both sides can theoretically recover into complete individuals.
COTS have a tremendous reproductive capacity: a single adult female can release up to 100 million gametes, with a high fertilization rate. During spawning period, which tend to occur during summer periods when water temperature is the warmest, stress – by being touched, manipulated or harmed – can prompt an individual to spawn, i.e. releasing its gametes. Moreover, it can trigger surrounding individuals to spawn at the same time. Larvae can drift up to 10 days in the water column before starting to roam on the seabed, and can theoretically thus be carried from a few kilometers to several hundreds kilometers away from their place of origin. Most larvae don’t survive into adulthood, which explains the relative low density of COTS on reefs under normal conditions. During the first 6 months, algal-feeding juvenile COTS of 1-10mm diameter are mostly found in deeper areas at the base of the reef slopes; then they gradually spread upwards to reach coral-rich areas where they usually hide under table corals, in crevices or sheltered spots during the day, making them hard to spot. Adults are similarly nocturnal, except during massive outbreaks where they can be easily seen anytime.
While larval stages mostly feed upon plankton, adults are exclusively corallivores: they feed on coral polyps by pushing their stomachs out of their mouth before releasing their digestive enzymes (extracorporeal digestion). Once the process is complete, only the calcareous skeleton of the coral remains. These food scars are recognized by their white color and often constitute the first evidence of COTS infestations; however they can easily be confused with bleaching resulting from other coral predators (e.g. mollusc Drupella sp.) or from various coral diseases.
COTS are a natural component of coral reef ecosystems. In a “healthy” reef, population density is usually low, ~1 to 10 individuals per hectare (100m x 100m), and has no detrimental effect on the abundance and diversity of coral assemblages. Instead, COTS contribute to maintaining a high coral diversity through its marked food preferences: fast growing, branching or sub-massive species (such as Acropora, Montipora or Pocillopora) are preferred over slower-growing, massive corals (such as Porites). Adult specimens are reported to consume up to 12 m² of coral per year. They have few regular predators except the Triton’s trumpet (Charonia tritonis); however recent studies increasingly report several fish (the stellate puffer Arothron stellatus, the humphead wrasse Cheilinus undulatus, and some triggerfish, emperor or boxfish) or invertebrate species occasionally preying on young and damaged or weakened adults crown-of-thorns. COTS longevity has to be confirmed yet; they probably can live up to 10-15 years in ideal conditions.
The acanthaster phenomenon
While A. planci generally occurs at very low densities, populations can dramatically increase during certain periods, eventually reaching extremely high values (e.g. several thousands or tens of thousands individuals per hectare). These outbreaks, which can persist for months or years over vast reef areas, represent one of the most significant biotic disturbances on coral reefs, causing massive and widespread coral mortality. Coral mortality can exceed 90% in the most severely affected reef areas, resulting in a profound restructuring of the ecosystem structure and ability to function.
Over a third of Indo-Pacific reefs were recently affected by severe episodes of COTS outbreaks, leading to growing concerns that they are becoming more frequent and more prevalent, and driving even more pressure on already weakened systems. The cascading effects from coral loss can severely harm the entire coral community, and sometimes lead to a phase-shift where the system becomes totally dominated by algae and sees little possibility of recovery in less than a few decades. This raises serious concerns in areas where coastal resources (fish, invertebrates) form the basis of traditional, subsistence fishing.
Despite sustained research efforts by the scientific community since the early 70s, the causes and dynamics of these outbreaks are still poorly understood. Some researchers believe that one factor could be the increasing scarcity of acanthasters’ natural predators due to the overfishing of commercial species such as tritons, humphead wrasses, emperors, etc. The overall deterioration in water quality, linked to human activities, is also highlighted, but this hypothesis alone cannot explain all outbreaks, especially those observed on unpolluted reefs. For other authors, these demographic explosions could be part of the biological cycle of the species, which is naturally predisposed to large fluctuations as a result of its extraordinary fertility. Recent scientific research suggests that none of these hypotheses alone can explain the highly complex, multifactorial and multi-scale nature of COTS.
“Acanthaster threat” and global change
In the Indo-Pacific region, the frequency and intensity of outbreaks appears to be increasing, especially in recent decades, with growing awareness of the global changes that are affecting the region. Acanthasters develop best in warm waters (26 to 30°C) in the presence of phytoplankton, and can therefore be particularly sensitive to the effects of climate change. Increased sea surface temperature and nutrient enrichment in coastal waters (in Southeast Sulawesi, this is caused notably by the runoff of sediments generated by mining activities or inland plantations) are considered major contributing factors to the survival of larvae, thus increasing the number of adults potentially reaching the reefs at the end of their development. Given the large dispersal capacities of the species, whose swimming larvae can settle several hundred kilometers from their original reef, the growing regional “acanthaster threat” is a real problem in the context of current climate change scenarios. Although there is historical evidence that coral reefs can recover after an outbreak, recovery is generally slow (several decades) and not guaranteed. Acanthaster outbreaks are yet another pressure on ecosystems, which are increasingly weakened by other natural (coral bleaching, tropical cyclones, coral diseases, etc.) and anthropogenic (pollution, overfishing, coastal developments, etc.) disturbances. For instance, in the Australian Great Barrier Reef, a recent study reports a 50% decline in coral cover over the last 30 years: half of the decline is due to recurrent acanthaster outbreaks alone.
Managing COTS outbreaks
In the majority of Pacific countries, where coral resources form the basis of traditional fisheries, massive acanthaster outbreaks can threaten the food security and livelihoods of coastal populations. The issue is also of concern to the tourism industry (diving clubs, hotels, etc.) whose activity can be seriously impacted by uncontrolled proliferation, and environmental managers for whom it is now a conservation issue.
Currently, only human actions can combat the proliferation of acanthasters, more or less successfully, depending on the extent of the phenomenon, the characteristics of the impacted reefs (size, isolation, vulnerability, etc.), the context (socio-economic, environmental) and the available resources (human, financial). Due to the already manifold threats on coral reefs worldwide – global warming, overfishing, trawling, coastal development, tourism, etc. – it is now widely accepted that alleviating the pressure caused by COTS outbreaks can improve reef resilience. Proactive management, e.g. taking direct action to control COTS populations when facing an outbreak is increasingly recommended throughout the Pacific region, especially if a reef in good condition is threatened.
Past attempts in managing COTS outbreaks
A variety of methods have been developed in the past to control COTS outbreaks, with limited success: direct removal using sticks, hooks or similar tools; cutting up or crushing up in situ; leaving on site in bags; construction of underwater fences; and more recently injections with a variety of chemicals or pathogens. In the Indo-Pacific countries, manual collection followed by disposal ashore is the most commonly used for small-scale cleanup efforts.
Yet, the efficiency of these measures is still controversial, in particular to protect entire reef systems: “success stories” tend to be small in scales, providing only short-term solutions to a complex phenomenon whose ultimate causes are not fully understood. Removal initiatives may be in particular challenged by the great ability of A. Planci to recolonize cleaned areas, which is modulated by biological and environmental factors interacting over a range of scales (e.g. reef extent and topography, local/regional connectivity, distance/structure of source populations, available human & financial resources etc.).
The injection method, being the most cost-effective, is increasingly replacing collection methods. It involves inoculating a toxic solution that causes the death of acanthasters. However, these treatments have some drawbacks, and the use of several chemicals (e.g. copper sulfate, sodium bisulfate, formaldehyde, ammonia, bleach, etc.) had to be discontinued due to their toxic effect on the environment and other species as well as their high cost.
The best solution today: acidic injections, a new “cheap and natural” alternative
A new approach based on the injection of natural and cheap acidic substances has recently been developed by the IRD. Tests were carried out both in the lab under controlled conditions and in the field and demonstrated the lethal properties of some fruit juices (different varieties of lime and passion fruit), white vinegar and some powdered acids from the agri-food industry. These substances, which cause high mortality even at low doses and 100% mortality reached within 12–24h, are now a highly credible ecological alternative in fighting acanthaster outbreaks for all countries. Tested by IRD since 2014 in Vanuatu and New Caledonia, it proved its efficiency in the field with for instance the eradication of more than one ton of acanthasters over two days.
A local management system: citizen monitoring networks & diving intervention task force
The unpredictability of these outbreaks makes their management particularly complex. As effective as they may be, these methods of control are merely a symptomatic treatment of the phenomenon. They require that any proliferation is detected at the earliest possible moment. This task may seem overwhelming for countries with extensive reef formations (such as Indonesia), as long-term monitoring requires considerable resources often not available to protected area managers and marine conservation organizations.
An interesting alternative is the creation of “citizen” monitoring networks, where data collection is provided by the local users themselves (mostly fishermen from nearby villages, possibly tourist guides and divers). Fishermen notably can easily survey every month a few locations near their fishing spots, with little impact to their schedule. Data is then passed on at the village level, then centralized in a single database shared by natural resources managers and marine conservation organizations. If one data point shows a number of COTS above a normal threshold (and confirmed by a second observation), a COTS outbreak warning is triggered. A task force composed of 5-6 scuba divers trained in the injection method is then dispatched on location to eradicate the outbreak.
This material was put together for a workshop held in 2021 in Kendari, Southeast Sulawesi, Indonesia, with local diving clubs and government agencies involved in natural resources management. It was put together by Pascal Dumas (IRD – Centre de Nouméa, New Caledonia) and Yann Bigant (Naturevolution) and translated into Indonesian by Mutmainnah (Naturevolution Indonesia).